Improvements for GAS testcase generation

[Doug Evans]

Geoffrey Keating writes:
 > This patch changes the GAS testcase generator.  The old generator used
 > to pick a few random small integers or a random keyword as operands.

The old generator was a quick hack to get something going pending
someone to come along and do something better.  [Isn't there something
in ESR's paper about not trying to do everything and leaving some
things for someone else to do? :-)]

 > OK to commit?

Yep, thanks!